1. Field of the Invention
This invention relates to improvements in the device for detecting focusing conditions of a principal image forming optical system on a trigonometrical basis.
2. Description of the Prior Art
The auto-focus apparatus of the type in which the focus detecting device projects onto an object to be photographed a non-harmonic light to the ambient light and receives the reflection of the non-harmonic light from the object, producing an output signal representing the distance from the device to the object, and a photographic lens is moved in response to a signal to focus an image of the object, or of the so-called active type, have been proposed in many forms. An early form of the active type auto-focus apparatus is shown in U.S. Pat. No. 3,442,193.
In this apparatus, the light projecting means and the reflection receiving means are positioned on the basis of the principle of trigonometrical survey, and operate without depending on the photographic lens optical system. To measure the object distance, therefore, special two optical systems must be respectively used for light projecting and receiving means. The use of such a not-through-the-lens or external range finder type focus detecting device leads to an increase in the size of the auto-focus apparatus.
Another type of focus detecting device is a non-harmonic light projection optical system which is provided outside of the photographic lens optical system, but the non-harmonic light reflection is sensed through the photographic lens optical system, as shown in U.S. Pat. No. 3,678,835, in the embodiment of FIG. 5. This type has an alternative problem of lowering distance measurement accuracy, because the base line length is necessarily short. On the other hand, the external range finder type shown in U.S. Pat. No. 3,442,193 is amenable to increasing the base line length to a desired value for the purpose of obtaining highly accurate distance measurement.
An object of the present invention is to provide a focus detecting device which has overcome the above-described problems and which improves focus detection accuracy without involving an increase in the size thereof.
To achieve this, according to the present invention, the optical axis of one of the two optical systems for measuring the object distance, which lies in the interior of a principal image forming optical system, is offset from the optical axis of the principal image forming optical system in a direction farther away from the optical axis of the other distance measuring optical system, so that the base line length is longer than the distance between the optical axes of the principal image forming optical system and the latter object distance measuring optical system.